EP3974636A1 - Method for controlling a bi-fuelled power system for a motor vehicle and associated processing unit - Google Patents

Abstract

The invention relates to a method for controlling a dual-fuel supply system of an internal combustion heat engine (2) for a motor vehicle (1) comprising at least a first injector (9) located in an intake duct (7) of at least one combustion chamber (6) of said engine, and a second injector (10) comprising an injection nozzle (11) located in said combustion chamber. This method comprises the injection of a first fuel by the first injector into the intake pipe to operate said engine and the simultaneous injection of a second fuel by the second injector into the combustion chamber is proposed.

The second fuel is injected when the temperature of the injection nozzle of the second injector is greater than a first threshold for a predetermined duration (Tp), the flow rate of the second injected fuel being determined to cool the injection nozzle so that the temperature of the injection nozzle is below a second threshold.

Description

The present invention relates to dual-fuel heat engines of motor vehicles, and more particularly to a method for controlling a dual-fuel supply system of a heat engine and a processing unit implementing the control method.

A motor vehicle internal combustion heat engine can be equipped with two separate fuel injection systems generally comprising a direct injection system and an indirect injection system to supply the engine with fuels of two different types.

The direct injection system introduces a first type of fuel, generally gasoline, into the combustion chambers of the engine, and the indirect injection system introduces a second type of fuel, generally liquefied petroleum gas LPG or liquefied natural gas LNG, upstream of the engine intake valves, for example in air intake pipes of the engine cylinder head or even in an engine intake distributor.

When the engine is fed by the direct injection system, injectors inject the first type of fuel into the combustion chamber.

Passing through each injector, the first fuel cools each of the injectors so that the temperature of the injectors remains below their maximum allowable temperature, preventing their deterioration.

When the engine is fed by the indirect injection system, the injectors of the direct injection system heat up under the effect of the temperature generated in the combustion chamber of the engine, but are no longer cooled by the first fuel.

The temperature of the injectors of the direct injection system increases sharply so that, under certain conditions, in particular on certain highly loaded operating points of the engine and after a certain period of time, the temperature of the nozzles of the injectors may come to exceed the maximum temperature allowable nozzles.

You can refer to the document KR2017/0124358 which discloses an engine comprising a direct injection system and an indirect fuel injection system.

When the engine is running on indirect injection and the temperature of the direct injection injectors becomes excessive, the indirect injection system is shut down and the direct injection system is started up to supply the engine exclusively.

The method disclosed makes it possible to lower the temperature of the injectors of the direct injection system.

However, when the engine is supplied with gasoline by the direct injection system, the fumes of combustion residues, in particular the fumes of carbon dioxide, are greater than those which are released when the engine is supplied by the injection system. indirect delivering a gaseous fuel.

Carbon dioxide emissions are about 10% higher when gasoline fueling is exclusive.

The document US2009/0292443 discloses a method for controlling the fuel supply to an engine, the method possibly comprising the injection of a first fuel by injectors of an indirect injection system and the injection of a second fuel by injectors of a direct injection system.

The simultaneous injection of fuel from the injectors of the direct and indirect injection systems makes it possible to reduce the temperature of the injectors of the direct injection system.

However, this document does not disclose a precise method for adjusting the quantity of second fuel to be injected in order to control the temperature of the injectors of the direct injection systems.

If the quantity of second fuel injected is too low, said injectors are insufficiently cooled, and if the quantity of second fuel injected is too high, the cooling is more than sufficient, but the carbon dioxide emissions increase unduly.

It is therefore proposed to overcome all or part of the aforementioned drawbacks and, in particular, to prevent the degradation of the injectors of the direct injection system while minimizing the emissions of combustion residues.

In view of the foregoing, the invention proposes a method for controlling a dual-fuel supply system of a thermal internal combustion engine for a motor vehicle comprising at least a first injector located in an intake duct of at least one combustion chamber of said engine, and a second injector comprising an injection nozzle located in said combustion chamber, the method comprising injecting a first fuel through the first injector into the intake pipe to operate said engine and the simultaneous injection of a second fuel by the second injector into the combustion chamber.

The second fuel is injected when the temperature of the injection nozzle of the second injector is greater than a first threshold for a predetermined duration, the flow rate of the second injected fuel being determined to cool the injection nozzle so that the temperature of the injection nozzle is below a second threshold.

• la détermination d'une première température de la buse d'injection à partir du régime et de la charge du moteur, et du mode de carburation du moteur,
• la détermination d'une deuxième température de la buse d'injection à partir de la première température de la buse d'injection et de la température d'un liquide de refroidissement du moteur,
• la correction de la deuxième température de la buse d'injection lorsque le moteur n'est pas alimenté en carburant, la température de la buse étant égale à la deuxième température de la buse ou à la deuxième température corrigée de la buse.

According to one characteristic, the determination of the temperature of the injection nozzle of the second injector comprises:

• the determination of a first temperature of the injection nozzle from the speed and the load of the engine, and the fuel mode of the engine,
• determining a second temperature of the injection nozzle from the first temperature of the injection nozzle and the temperature of an engine coolant,
• the correction of the second temperature of the injection nozzle when the engine is not supplied with fuel, the nozzle temperature being equal to the second nozzle temperature or the second corrected nozzle temperature.

Preferably, at least a first map links the first temperature of the injection nozzle to the speed and to the load of the engine, according to the fuel mode of the engine.

Advantageously, at least one second map links a correction coefficient to the temperature of the engine coolant, the second temperature of the injection nozzle being equal to the correction coefficient multiplied by the first temperature of the injection nozzle.

According to another characteristic, the determination of the second corrected temperature of the nozzle comprises the multiplication of the second temperature of the nozzle by a first order filter modeling the temporal evolution of the temperature of the nozzle when the motor is not supplied in fuel.

Preferably, the flow rate of the second fuel injected is determined from the temperature difference between the temperature of the nozzle and a maximum admissible temperature of the nozzle, and from the speed and the load of the engine.

Advantageously, at least a third map links the flow rate of the second injected fuel to the temperature difference, and to the engine speed and load.

Preferably, when the second fuel is injected, the flow rate of the first fuel injected is reduced and the flow rate of the second fuel is adjusted so that the richness of the mixture of air and of the first and second fuels is equal to 1.

Advantageously, the temperature of the injection nozzle of the second injector is determined at each engine cycle.

The invention also relates to a processing unit for a dual-fuel supply system of an internal combustion heat engine for a motor vehicle.

The processing unit is configured to determine a flow rate of a second fuel and inject said flow rate, the second fuel being intended to be injected by at least one second injector comprising an injection nozzle located in at least one combustion chamber of the engine when at least one first injector located in an intake pipe of the combustion chamber injects a first fuel into the intake pipe to operate said engine, the second fuel being injected simultaneously with the first fuel when the temperature of the injection nozzle of the second injector is greater than a first threshold for a predetermined duration, the flow rate of the second injected fuel being determined to cool the injection nozzle so that the temperature of the injection nozzle is below a second threshold.

• [Fig 1] illustre schématiquement un véhicule automobile doté d'une unité de traitement selon l'invention,
• [Fig 2] illustre un exemple de modélisation de l'évolution de la température de la buse d'injection du deuxième injecteur en fonction du débit de deuxième carburant selon l'invention,
• [Fig 3] illustre un exemple de modélisation de l'évolution du débit de deuxième carburant en fonction de la température de la buse d'injection du deuxième injecteur selon l'invention, et
• [Fig 4] illustre un exemple de procédé de commande du système d'alimentation à bicarburation selon l'invention.

Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawings in which:

• [ Fig 1 ] schematically illustrates a motor vehicle equipped with a processing unit according to the invention,
• [ Fig 2 ] illustrates an example of modeling the evolution of the temperature of the injection nozzle of the second injector as a function of the flow rate of the second fuel according to the invention,
• [ Fig.3 ] illustrates an example of modeling the evolution of the second fuel flow as a function of the temperature of the injection nozzle of the second injector according to the invention, and
• [ Fig 4 ] illustrates an example of a method for controlling the dual-fuel supply system according to the invention.

The figure 1 illustrates a motor vehicle 1 comprising a dual-fuel internal combustion heat engine 2, and a dual-fuel supply system for the engine 2 comprising an engine control device 3 and an injector cooling device 4.

In what follows, it is considered that the engine 2 comprises a cylinder 5 comprising a combustion chamber 6 fed by an intake duct 7 and connected to an exhaust duct 8.

Of course, the engine 2 can comprise more than one cylinder, the cylinders being identical.

The engine 2 comprises an indirect fuel injection system comprising a first injector 9 located in the intake duct 7, and a direct fuel injection system comprising a second injector 10 equipped with an injection nozzle 11 located in the intake pipe 7.

The intake duct 7 further comprises a throttle body 12 upstream of the first injector 9 to regulate the flow of air admitted into the combustion chamber 6.

The engine 2 further comprises a device for measuring the richness of the air/fuel mixture comprising at least one oxygen sensor 14 located in the exhaust pipe 8, in particular upstream of a pollution control device of the three-way catalyst type (not shown) in the figure.

Preferably, the oxygen sensor 14 is a lambda sensor (that is to say of the proportional type).

The first injector 9 delivers a first fuel, for example a gaseous fuel such as liquefied petroleum gas LPG or liquefied natural gas LNG or a fuel of the liquid type, for example gasoline, and the second injector 10 delivers a second fuel, for example a liquid fuel such as gasoline or a fuel of the gaseous type, for example liquid LPG.

It is assumed for example in what follows that the first fuel comprises a gaseous fuel and the second fuel comprises gasoline, which corresponds for example to the case of numerous engines operating on LPG placed on the market.

An inlet valve 15 located between the first injector 9 and the combustion chamber 6 regulates the admission of the air/first fuel mixture or air into the combustion chamber 6.

Motor 2 is cooled by a cooling liquid.

The engine control device 3 controls the engine 2, in particular the first and second injectors 9, 10 and the throttle body 12 as a function in particular of the richness measuring device, of instructions from the driver of the vehicle 1 provided for example by the pedal vehicle accelerator, and the injector cooling device 4.

The engine control device 3 comprises a first injection controller 16 controlling the first injector 9 and a second injection controller 17 controlling the second injector 10, which can be combined with each other.

The injector cooling device 4 comprises a processing unit 18 and a memory 19 connected to the processing unit 19.

The processing unit 18 determines a flow rate Qp of second fuel intended to be injected by the second injector 10 simultaneously with a flow rate of the first fuel when the temperature T-11 of the injection nozzle 11 of the second injector 10 is greater than a first threshold SI for a predetermined duration Tp.

The flow of second fuel is determined to cool the nozzle 11 so that the temperature of the nozzle 11 is lower than the first threshold S1.

The second fuel flow Qp cools the injection nozzle 11 when the combustion chamber 6 is supplied by the first injector 9.

Maps 20, 21, 22, a transfer function H(P) of a first order filter 23, the first threshold SI and the predetermined duration Tp are saved in the memory 19. The interest of the maps and of the function transfer is specified below.

The first threshold SI is for example determined from the maximum allowable temperature Tmax-11 of the nozzle 11 to operate without degrading, the maximum allowable temperature Tmax-11 generally being provided by the manufacturer of the injector 10.

A first map 20 links a first temperature Temp1 of the injection nozzle 11 of the second injector 10 to the speed and to the load of the engine 2 according to the fuel mode of the engine.

The load of engine 2 corresponds in a manner known per se to the torque delivered by the engine or to the average effective pressure of cylinder 5.

A second map 21 linking a correction coefficient COEF to the temperature of the engine coolant 2 is saved in the memory 19.

Taking into account the temperature of the coolant makes it possible to estimate the temperature of the nozzle 11 more precisely than by the first mapping alone, the coolant of the nozzle 11 cooling the nozzle 11 via the intermediary of the engine cylinder head 2.

A third map 22 connects a flow rate of the second fuel to a temperature difference and to the engine speed and load.

The third map 22 takes into account the cooling of the nozzle 11 generated by the passage of gasoline through said nozzle.

The determination of the first, second and third maps is detailed in the following.

The transfer function H(p) models the time evolution of the temperature of the nozzle 11 when the engine 2 is not supplied with fuel, for example when the driver of vehicle 1 completely releases the accelerator pedal of vehicle 1 , for example on a downhill slope to benefit from engine braking.

The complete release of the accelerator pedal is interpreted by the engine control device 3 as a zero setpoint of the torque delivered by the engine 1 so that the engine control device 3 cuts off the fuel injection, the device 3 controlling in generally besides the throttle housing 12 so as to close the throttle to maximize engine braking.

The absence of combustion in the combustion chamber 6 and the scavenging of the cylinder 5 by air flowing through the closed throttle body 12 cools the nozzle 11.

The time constant of the transfer function H(p) is determined from readings of cooling duration measurements of the nozzle 11 carried out on a test bench when the engine 2 passes from an operating point where it is supplied by the first injector 9 to a point of zero fuel flow.

For each stabilized operating point of the engine 2 defined by a speed and load pair, and for different stabilized known temperatures of the nozzle 11, the accelerator pedal is completely released.

The maximum cooling time of the nozzle is equal to the time necessary for the temperature of the nozzle 11 to stabilize at a temperature close to that of the cooling liquid.

The steps for obtaining the three maps 20, 21 and 22 are now detailed.

To obtain the first map 20, one begins with a first phase in which the temperature of the nozzle 11 is raised for each operating point of the engine 2 when said temperature is stabilized, the engine 2 being supplied with fuel by the second injector 11 in order to to establish a first part of the first stabilized temperature map.

In addition, in a second phase, the temperature of the nozzle 11 is raised for each operating point of the engine 2 when said temperature has stabilized, the engine 2 being supplied with fuel by the first injector 9 in order to establish a second part of the first stabilized temperature map.

When the first and second parts of the first map of stabilized temperature are established, we pass to a third phase in which, for each operating point, the engine 2 is first of all supplied by the second injector 10. When the temperature of the nozzle 11 has reached the stabilized value corresponding to said operating point determined by the first stabilized temperature map, the second injector 10 is cut off and the first injector 9 is actuated to supply the engine 2. The switchover from the first to the second injector s is done brutally.

Then, the evolution over time of the temperature of the nozzle 11 is recorded until the temperature of the nozzle 11 reaches the corresponding temperature of the second stabilized temperature map. A third part of the map is thus obtained which gives the temperature of the nozzle 11 during a transient phase of the carburetion mode.

In summary, the first map 20 makes it possible to estimate the temperature of the nozzle 11 as a function of the carburetion mode of the engine 2, that is to say, when the engine is supplied with fuel exclusively by the first injector (mode d indirect injection) or exclusively by the second injector (direct injection mode) and according to the duration of a changeover from indirect injection mode to direct injection mode or vice versa (transient mode).

In order to determine the first map 20 more precisely, the evolution over time of the temperature of the nozzle 11 can be noted in a similar manner during the passage from the indirect supply mode to the direct supply mode.

Temperature and time measurements can be performed on a test bench.

Now the determination of the second mapping 21 is detailed.

In order to simplify the determination of the second map 21, a digital thermal model of the cylinder head of the engine 2 is produced.

The second map 21 is determined by measuring the temperature of the nozzle 11 calculated by the digital model for different temperatures of the coolant, the coefficient COEFF being determined by varying the cooling temperature for an engine operating point, the simulations being repeated for each operating point.

As a variant, the determination of the coefficient COEFF can be carried out from measurements carried out on an engine test bench.

This coefficient is used to correct in a simple manner the values of the temperature of the nozzle 11 resulting from the first mapping. In alternatively, it is also possible to make all the measurements of the first map by scanning the various possible values of coolant temperature in addition to the speed and load values. It is more tedious.

The third map aims to know the temperature of the nozzle 11 that can be obtained if, instead of operating the engine in exclusive indirect injection mode, a proportion of fuel injected in direct injection mode is combined.

To determine the third map 22, a map of the temperature of the nozzle 11 in at least partially indirect injection mode is determined by noting the temperature of the nozzle 11 during the variation of the flow rate of second fuel injected by the second injector 10 for each engine operating point, engine 2 operating in at least partially indirect injection mode.

The temperature readings are for example taken on an engine test bench.

pour D1, et $$\mathrm{y}=\mathrm{A}2\mathrm{x}+\mathrm{B}2$$

pour D2,
les coefficients A1, B1, C1 et D1 étant des constantes.From the temperature readings obtained and for each engine operating point, the evolution of the temperature of the nozzle 11 as a function of the flow rate of the second fuel injected is modeled by two straight lines D1 and D2 having the equation: $$\mathrm{there}=\mathrm{AT}\mathrm{1}\mathrm{x}+\mathrm{B}\mathrm{1}$$

for D1, and $$\mathrm{there}=\mathrm{AT}2\mathrm{x}+\mathrm{B}2$$

for D2,
the coefficients A1, B1, C1 and D1 being constants.

The figure 2 illustrates for a given operating point the evolution of the temperature T-11 of the nozzle 11 as a function of the second fuel flow Q2, and the straight lines D1 and D2 modeling said evolution of the temperature T-11 of the nozzle 11.

A temperature gain to be achieved so that the temperature of the nozzle 11 is less than or equal to the first threshold SI in at least partially indirect injection mode is determined by determining a temperature gain map of the nozzle 11. The gain map is determined by calculating the difference between each temperature of the first stabilized temperature map and the first threshold S1.

pour D3, et $$\mathrm{y}=\mathrm{C}\mathrm{2}\mathrm{x}+\mathrm{D}2$$

pour D4
les coefficients C1, D1, C2 et D2 étant des constantes.From the straight lines D1 and D2 and the temperature gain map, for each operating point of the engine 2, the evolution of the flow rate of second fuel Q2 as a function of the temperature gain of the nozzle 11 is modeled by two straight lines D3 and D4 having the equation: $$\mathrm{there}=\mathrm{VS}\mathrm{1}\mathrm{x}+\mathrm{D}\mathrm{1}$$

for D3, and $$\mathrm{there}=\mathrm{VS}\mathrm{2}\mathrm{x}+\mathrm{D}2$$

for D4
the coefficients C1, D1, C2 and D2 being constants.

The third map 22 includes all of the coefficients C1, D1, C2 and D2 for each operating point.

The third map 22 makes it possible to precisely determine for each operating point a flow rate of second fuel to be injected in order to reach the temperature gain of the nozzle 11.

The fuel flow to be injected by the second injector 10 to reach the determined temperature gain corresponds to the maximum of the values determined by the straight lines D3, D4.

The picture 3 illustrates for a given operating point the evolution of the second fuel flow rate Q2 modeled by the straight lines D3, D4 as a function of the temperature T-11 of the nozzle 11.

The figure 4 shows an example of a dual-fuel fuel system control method.

It is assumed that the first, second and third maps 20, 21 and 22, the transfer function H(P) of the filter 23, the first threshold SI and the predetermined duration Tp are saved in the memory 19.

It is assumed that a switchover from the direct supply mode to the indirect supply mode occurs, the switchover instant being saved by the processing unit 18.

For example, on an engine running on gasoline with direct injection and LPG with indirect injection, this happens a few seconds after starting the gasoline engine, in particular for legal reasons. In the rest of the journey of the vehicle, if the quantity of LPG fuel in the tank concerned allows it, the engine control device 3 tends to favor the operating mode of the engine in indirect injection mode of LPG which minimizes the emissions of pollutants .

During a step 30, the processing unit 18 determines a first temperature of the nozzle 11 from the first map 20, the speed and the load of the engine 2, and the time elapsed since the moment of the mode switchover. direct feed mode to indirect feed mode.

The engine speed and load 2 are for example transmitted to the processing unit by the engine control device 3.

Then, in a step 31, the processing unit 18 determines the value of the correction coefficient COEF from the temperature of the coolant provided for example by the engine control device 3. The processing unit 18 determines a second temperature of injection nozzle 11 by multiplying the correction coefficient COEFF by the first temperature of nozzle 11.

At this stage, the temperature of the nozzle 11 is known while the engine is operating without any injection of fuel through said nozzle. The temperature of nozzle 11 tends to increase with time spent in pure indirect injection mode. The temporal evolution profile of said temperature depends on the regime-load operating point of the engine, which depends in particular on the depression of the accelerator pedal of the vehicle.

However, it is also possible that, while the engine is operating in this indirect injection mode, the driver suddenly takes his foot off the accelerator pedal, for example if the vehicle is entering a downward slope on which the driver wishes benefit from engine braking. Fuel injection cut-off then tends, on the contrary, to cool the temperature of the injectors. The method takes account of such an event to correct the evaluation of the temperature of the nozzle 11.

If the engine 2 is not supplied with fuel (step 32), the processing unit 18 multiplies the second temperature by the first order filter 23 (step 33), the temperature T-11 of the nozzle 11 being equal to the second temperature multiplied by the first order filter 23.

If the engine 2 is supplied with fuel (step 32), the temperature T-11 of the nozzle 11 is equal to the second temperature.

The motor control device 3 provides, for example, motor power information.

In a step 34, the processing unit 18 compares the temperature T-11 of the nozzle 11 with the first threshold S1.

The first threshold SI is for example equal to the maximum admissible temperature Tmax-11 plus a hysteresis value.

If the temperature T-11 of the nozzle 11 is lower than the first threshold SI (step 34), the engine 2 remains supplied by the first injector 9.

If the temperature T-11 of the nozzle 11 is higher than the first threshold SI (step 34) for the predetermined duration Tp, the processing unit 18 determines the flow rate Q2 of second fuel to be injected to cool the second injector 11 from the temperature T-11 of the nozzle, the speed and the load of the engine 2, and the third map 22 (step 35).

The hysteresis makes it possible to avoid untimely switching when the operating point of motor 2 is around a boundary zone between the indirect engine supply mode and the dual-fuel supply mode of engine 2 (simultaneous injection of the first and second fuels into the combustion chamber 6).

The hysteresis value is for example determined by calibration tests on vehicle 1.

Then, in a step 36, the processing unit 18 transmits the second fuel flow rate Q2 to the engine control device 3.

The first injection controller 16 controls the first injector 9 and the second injection controller 17 controls the second injector 10 so that the second injector 10 injects the flow Q2 of second fuel, the flow of first fuel injected by the first injector 9 being injected so that the richness setpoint of the air mixture and the first and second fuels is equal to 1.

The engine 2 is supplied by the first and second injectors 9, 10 simultaneously in dual-fuel supply mode (“dual-fuel” mode).

For example, the flow of first fuel can be injected in an open loop, and the flow of second fuel is adjusted by adding to a prepositioning value an injection quantity correction capable of regulating the richness around the stoichiometric value.

If the temperature T-11 of the nozzle 11 is lower than a second threshold S2 equal for example to the maximum admissible temperature Tmax-11 minus the hysteresis value (step 37), the processing unit 18 controls the device 3 from so that the second injector 10 is cut off.

The first injection controller 16 drives the first injector 9 so that the richness setpoint of the air and first fuel mixture is equal to 1, the temperature of the nozzle 11 being lower than the maximum allowable temperature (step 38). The process then continues at step 30.

If the temperature T-11 of the nozzle 11 is greater than the second threshold S2, the method goes to step 36.

The processing unit 18 determines the temperature T-11 of the nozzle 11, and if necessary determines and injects the flow rate Q2 of second fuel at regular time intervals, for example at each injection cycle of the first fuel equal for example to 10 ms.

The condition on the predetermined duration Tp before switching to dual-fuel supply mode makes it possible to filter out a rapid transient phase from the estimation of the temperature of the nozzle 11.

The second threshold S2 makes it possible to inject the second fuel to cool the nozzle 11 to a temperature lower than that of the first threshold S1 to prevent incessant switching of the supply mode as the temperature of the nozzle 11 evolves.

The control method of the bi-fuel supply system makes it possible to inject the necessary quantity of second fuel to cool the nozzle 11 so that it does not deteriorate while reducing the emissions of polluting residues, in particular carbon dioxide.

Claims (10)

Method for controlling a dual-fuel supply system of an internal combustion heat engine (2) for a motor vehicle (1) comprising at least one first injector (9) located in an intake duct (7) of at least one combustion chamber (6) of said engine, and a second injector (10) comprising an injection nozzle (11) located in said combustion chamber, the method comprising the injection of a first fuel by the first injector in the intake pipe to operate said engine and the simultaneous injection of a second fuel by the second injector into the combustion chamber, characterized in that the second fuel is injected when the temperature of the injection nozzle of the second injector is greater than a first threshold for a predetermined time (Tp), the flow rate of the second injected fuel being determined to cool the injection nozzle so that the temperature of the injection nozzle is below a second threshold. Method according to claim 1, in which the determination of the temperature of the injection nozzle (11) of the second injector (10) comprises: - the determination of a first temperature of the injection nozzle from the speed and the load of the engine (2), and the fuel mode of the engine, - the determination of a second temperature of the injection nozzle from the first temperature of the injection nozzle and the temperature of an engine coolant, - the correction of the second temperature of the injection nozzle when the engine is not supplied with fuel, the temperature of the nozzle being equal to the second temperature of the nozzle or to the second corrected temperature of the nozzle. Method according to claim 2, in which at least a first map (21) relates the first temperature of the nozzle injection to the engine speed and load (2), depending on the engine fuel mode. Method according to one of Claims 2 and 3, in which at least a second map (21) links a correction coefficient to the temperature of the coolant of the engine (2), the second temperature of the injection nozzle being equal the correction coefficient multiplied by the first temperature of the injection nozzle. A method according to any of claims 2 to 4, wherein determining the second corrected nozzle temperature comprises multiplying the second nozzle temperature by a first order filter (23) modeling the time evolution of the nozzle temperature when the engine (2) is not supplied with fuel. A method according to any one of claims 1 to 5, wherein the flow rate of the injected second fuel is determined from the temperature difference between the temperature of the nozzle (11) and a maximum allowable temperature of the nozzle, and engine speed and load (2). Method according to claim 6, in which at least a third map (22) links the flow rate of the second fuel injected to the temperature difference, and to the speed and to the load of the engine (2). Method according to one of Claims 1 to 7, in which when the flow rate of second fuel is injected, the flow rate of injected first fuel is reduced and the flow rate of the second fuel is adjusted so that the richness of the mixture of air and first and second fuels is equal to 1. Method according to one of Claims 1 to 8, in which the temperature of the injection nozzle (11) of the second injector (10) is determined at each engine cycle. Processing unit (18) for a dual-fuel supply system of an internal combustion heat engine (2) for a motor vehicle (1), characterized in that the processing unit is configured to determine a flow rate of a second fuel and inject said flow rate, the second fuel being intended to be injected by at least one second injector (10) comprising an injection nozzle (11) located in at least one combustion chamber (6) of the engine when at least one first injector located in an intake pipe (7) of the combustion chamber injects a first fuel into the intake pipe to operate said engine, the second fuel being injected simultaneously with the first fuel when the temperature of the injection nozzle of the second injector is higher than a first threshold for a predetermined duration (Tp), the flow rate of the second injected fuel being determined to cool the injection nozzle so that the temperature of the injection nozzle is lower than a second threshold.

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